1. Field of the Invention
The present invention relates to a diazonium salt used for a color-forming layer of a thermosensitive recording medium. More particularly, it relates to a diazonium salt which develops a red or yellow color when it reacts with a coupler, dissolves to a high concentration in capsule oil when sealed together with capsule oil in a microcapsule, and provides a thermosensitive recording medium that forms a high density, high contrast thermosensitive image and can prevent forgeries.
2. Description of the Related Art
In recent years, a thermosensitive recording medium using a diazonium salt which undergoes photodecomposition is replacing conventional thermosensitive recording media using color-forming leucodyes. The thermosensitive recording medium has a color-forming layer including a diazonium salt, coupler and basic compound on a support. It is known that, to improve the keeping properties of the thermosensitive recording medium, the diazonium salt is sealed together with capsule oil in a microcapsule by interface polymerization and is separated by the microcapsule wall from the coupler and basic compound so that it does not develop a color before exposing to heat. The thermosensitive recording medium using a diazonium salt is disclosed, for example, "Microcapsules" by Tamotsu Kondo, pp. 65-75, ed. Japan Standards Association (1991).
However, while many thermosensitive recording media using diazonium salts are known which develop a blue color, hardly any are known which develop a red or yellow color. In other words, although it also depends on the coupler, there are many diazonium salts which develop a blue color when they react with a coupler, and very few which develop a red or yellow color.
When the diazonium salt is sealed in the microcapsule, the diazonium salt must be dissolved in an oil (referred to hereafter as capsule oil) which forms the core of the capsule in order to form a stable emulsion which has a uniform particle diameter.
However, conventional diazonium salts have a low solubility in capsule oils such as dibutyl phthalate or tricresyl phosphate. For example, 4-diazodiphenyletherhexafluorophosphate, which is generally used for developing a red color, has a solubility of 0.5 g! or less in 100 g! of dibutyl phthalate or 100 g! of tricresyl phosphate. Further, even 4-diazo-4'-methyldiphenyletherhexafluorophosphate, which is generally said to have a high solubility in capsule oil, has a solubility of 1.0 g! or less in 100 g! of dibutyl phthalate or 100 g! of tricresyl phosphate.
A method was therefore proposed for adding a polar solvent such as acetone to raise the solubility of a diazonium salt such as 4-diazodiphenyletherhexafluorophosphate or 4-diazo-4'-methyldiphenyletherhexafluorophosphate in capsule oil in Japanese Patent Application No. 36215/1996.
However, as the polar solvent is also highly soluble in water, when the capsule oil, in which the diazonium salt is dissolved, is dispersed in water giving an aqueous emulsion, the diazonium salt tended to move into the aqueous phase and precipitated so that a concentration of the diazonium salt in the microcapsule fell. It is therefore necessary to dissolve a high concentration of the diazonium salt in the capsule oil alone without adding a polar solvent.
Hence, a diazonium salt, which is sealed together with capsule oil in a microcapsule and used for a color-forming layer of the thermosensitive recording media, was desired having the following properties:
1! Develops a red or yellow color when it reacts with a coupler, PA1 2! Has a high solubility in capsule oil, specifically a solubility of 1 g! or higher and preferably of 3-30 g! per 100 g! of capsule oil, and is capable of forming a high density, high contrast image, and PA1 3! provides a thermosensitive recording medium which can prevent forgeries.
In general, conventional diazonium salts are costly, and as they have poor thermal stability, their use in thermosensitive recording media was limited. A less expensive diazonium salt which had good heat withstanding properties was therefore required.